Scope pattern problems with new antenna

[VE7 Kilohertz]

 Hi Gang,

Just finished putting up my new antenna at my new QTH. It's a 160M dipole at about 60', fed with homebrewed ladder line, 4" spacing and using my balanced/balanced tuner.  I had previously used a 160M full wave loop for all bands with good sucess. I used a homebrewed balanced/balanced antenna tuner and had great scope patterns. I am using a "T" capacitively coupled sampler in the main coax line to feed channel 2 of my Tek 465 scope. Channel 1 is the IF out from the 6790GM.

So, now, with the new antenna, and everything else is the same, I am getting crappy looking modulation patterns on the "T" sample signal from the coax. No negative modulation showing and wierd looking positive peaks. The IF out from the 6790 look fine as do the TFT mod monitor meters. The TFT confirms over 100% positive and not quite 99% negative.

Audio sounds great on the 6790.

TX is my Bauer 707 putting out 750W.

So, am I getting some strange loading that is affecting the scope pattern? Is the dipole (2 wavelengths on 80M) causing some strangness?? I am doing all these tests on 80M.  The pattern looks great feeding a dummy load so that eliminates everything but the tuner and antenna.  I have everything in the station bonded to a common 3" wide copper strap which goes outside but is not yet connected to a ground rod.

Help!!

Cheers

Paul

[WA1GFZ]

Paul,
Put a 50 ohm load on the scope input. Sounds like common mode crud getting into your scope. Also make sure the tuner is well grounded. fc

[VE7 Kilohertz]

Hi Frank,

Thanks. I am feeding the scope directly with coax from the "T" in the main coax line. Should I just add another T at the scope input and hang 50 ohms on one leg of the T?

Cheers

Paul

[WA1GFZ]

No it will blow up. That is a lot of voltage to be feeding into a scope.
You might be better off builting a pickup. The stub hanging off the transmission line could also be a problem. I misunderstood your set up.
I would use a little mini box and cap couple off the feed line or build a directional coupler with a transformer.

[flintstone mop]

You lucky guys in CANADA get to have real RF power. We play with numbers here in the USA and were buffaloed in this PEP thing of 1500 W.
Fred

[KF1Z]

You lucky guys in CANADA get to have real RF power. We play with numbers here in the USA and were buffaloed in this PEP thing of 1500 W.
Fred

It could be worse.....
We could be in Ausralia, where they're limited to 100 watts....
(not sure if that's carrier, pep, or inpoot)

[k4kyv]

You lucky guys in CANADA get to have real RF power. We play with numbers here in the USA and were buffaloed in this PEP thing of 1500 W.
Fred

I thought the USA was supposed to be the country with the most freedom in all the world.  At least that's what I was taught in school.

Regarding the scope pattern, in the distant past I have had weird looking envelope patterns with parallel stripes all over the waveform.  I finally figured out that it was higher order harmonics, maybe 4th or 5th, that were riding in along with the fundamental.  I don't know where they were coming from, but when I changed to inductive coupling from the feedline,  they went away and the pattern was clean.

[AB2EZ]

Another possible source of this problem:

If the rig doesn't have a "safety choke" at the output, then the modulating audio will feed through the plate coupling capacitor and the tank circuit to the antenna port of the transmitter. If the monitor circuit (capacitive coupling in this case) is at a high impedance point (at audio frequencies), the signal picked up by the monitor will be the sum of the modulated rf and the modulating audio. This will produce the result you described.

Remember, even if the plate coupling capacitor is .001 uF, at audio frequencies it is forming a voltage divider with the parallel combination of the tuning capacitor (~200-500 pF) and the loading capacitor (~500-1500 pF). Thus a large portion of the audio voltage will appear across the rf output port of the transmitter, unless there is some kind of low impedance path to ground for the audio signal across the rf output of the rig.

Try placing an rf choke across the input of the scope to provide a low impedance path to ground for the audio.

Whenever you build a pickup that uses a capacitive voltage divider or a resistive voltage divider to work with a plate modulated rig, it is a good idea to include an rf choke to provide a low impedance path to ground for audio frequencies.

I have placed "safety chokes" at the outputs of some of my plate modulated rigs (e.g. my Johnson Ranger) to not only provide safety, but also to remove unwanted audio from the output of the rig. I find that 10 turns of insulated hookup wire on one of the type #43 ferrite cores that we use to make transformers for Class E rigs (1" outer diameter, 1 inch long, with a 1/4" wall thickness) works great for power levels below 50 watts (at carrier). For my homebrew legal limit linear amplifier, I used 6 turns of #14 insulated wire, wound around four (4) of these ferrite cores. You need more cores for a higher power safety choke in order to keep the magnetic fields inside the core from reaching levels that will cause the cores to overheat (from hysteresis losses).

Stu

[VE7 Kilohertz]

Hi Guys,

Thanks for the ideas.

I put another T in the line to the scope inout and out a 50 ohm load on one leg of the T and it solved the scope display problem. Yeah!!! However, in the last day, something else has now changed and now I have TONS of RFI....!!!$$%@#^&^@

I have removed everything new in the last day, gone right back to where it ws on Thursday night and i STILL have RFI getting into the audio. No positive peaks at all above the carrier.

Man oh man.

Back to the books...and the drawing board....and square one.

Maybe I will extend the dipole to .64 wavelengths on each half so that it becomes a extended double Zepp.... maybe I have feedline radiation which is only 4 feet from audio gear.

Sure miss my old 24' x 24' shack.  This one is 7'x 12'.

The Bauer monitor tap is busy feeding the TFT mod monitor. 
Yup, common mode chokes work well. Fixed many problems for me in the past.

Cheers
Paul

[AB2EZ]

Paul

Attached is a schematic of the type of balanced/balanced tuner that a lot of the folks on this board are using. This schematic was provided to me by K1KBW (Bob).

Note that the capacitor. C2 on the input (coax) side eliminates the path to ground for audio frequencies that you would have with a 50 ohm resistive dummy load attached to your transmitter.

This is probably why you had the problem. The 50 ohm load you placed across the input to the scope fixed the problem because it provides a 50 ohm path to ground for the plate modulating audio signal that is coupled through your plate coupling capacitor  / tank circuit to the output of your rig... and from there, through your pickup, to the scope.  If you don't have a "safety choke" across the output of your rig, your plate coupling capacitor forms a voltage divider with C2 that places a large fraction of the audio modulating signal across C2. This will cause a rather strange looking waveform to be displayed on your scope, unless your pickup is inductively coupled, or unless it provides a low impedance (e.g., a 50 ohm load or an rf choke) audio frequency path to ground... which will effectively remove the audio component.

For boat anchor applications, it would be a good idea to put a safety choke across the coaxial input to the tuner (center conductor to ground/shield). This will avoid erroneous readings on SWR meters, modulation monitors, scopes, etc. (unless they have their own, internal, low impedance paths to ground for audio frequencies).

I'm sorry to hear about your new RFI problem. I suggest that you check to be sure that all of the shield connections of the coaxial cables that run between your rig and the tuner are making good contact. If one of the coax shields comes loose, then the current that normally flows on the inside of this shield will find an alternate path... typically creating a big ground loop that radiates RF into the shack. If you have the RFI problem with a dummy load connected to the rig at the point where the tuner is... then that would suggest a loose shield problem. If the RFI problem does not occur wth the dummy load connected... but reappears when you remove the dummy load and reconnect the tuner... then the RFI is probably being caused by some kind of imbalance in the tuner that is allowing the balanced feeder line to radiate.

Best regards
Stu

[Steve - WB3HUZ]

You can see the original of the schematic and the associated article on The AM Window Web Site. The image there is much cleaner than the one below. I see someone (who doesn't understand image files) saved it as a JPEG and created a bunch of smearing.

http://www.amwindow.org/tech/htm/160smallants.htm

[w5omr]

I put another T in the line to the scope inout and out a 50 ohm load on one leg of the T and it solved the scope display problem. Yeah!!! However, in the last day, something else has now changed and now I have TONS of RFI....!!!$$%@#^&^@

Forget about all that stuff, Paul.

The scope has BNC's for the input, right?  Just take a hunk of coax with a BNC on it, connect to the scope, and take the other end outside, into the antenna field and monitor what's being radiated.  Keeping the output line of the transmitter clean, and one piece (and fewer connections = less places for failure to occur).

Monitor what's radiated - not what's generated in the final.

[VE7 Kilohertz]

I put another T in the line to the scope inout and out a 50 ohm load on one leg of the T and it solved the scope display problem. Yeah!!! However, in the last day, something else has now changed and now I have TONS of RFI....!!!$$%@#^&^@

The scope has BNC's for the input, right?  Just take a hunk of coax with a BNC on it, connect to the scope, and take the other end outside, into the antenna field and monitor what's being radiated.  Keeping the output line of the transmitter clean, and one piece (and fewer connections = less places for failure to occur).

Monitor what's radiated - not what's generated in the final.


I like this idea!!  Makes sense. I will give it a try, although the current way I have it connected is working again with the 50 ohm load on the input. 

Thanks guys. Going to post another question in a different thread about Extended Double Zepps.

Cheers

Paul

[WA1GFZ]

You might need to load the coax at 50 ohms on the scope input.
Outside I would make a pick up loop with a 50 ohm series resistor.

[WD8BIL]

Might simply be RF getting into the horizontal sweep amp in the scope roaching the waveform.
I gots the same problem on 160 meters with my Tek.
Moving the scope to a new location might help.

[nq5t]

I thought the USA was supposed to be the country with the most freedom in all the world.  At least that's what I was taught in school.

You must not ever read the newspaper or watch the TV news.  "Freedom" has gone the way of the gooney bird, from government meddling and the "Patriot" (give me a break!!) Act to those political groups calling themselves "God" inspired, who's heads have never seen the light of day and who believe that only their enlightenment is "true and righteous" and the rest of us better tow THEIR line ...

Just last night there was a news bit about the TSA now having "guidelines" on how to pack our suitcases (fold your clothes, bundle your power cords, pack in their "approved layer format").  Not to mention having to get the control freaks from the "neighborhood assiciations" involved if you want to paint your front door.

Next time I travel I'm packing my suitcase full of dirty clothes in a jumble, and plan to include a goodly layer of "used" skivvies on top with an appropriate degree of vintage ripe.   I'll be more than happy to let the b*stards rifle through the pile.  I can have the clothes washed when I get where I'm going  :-)

[KB2WIG]

Are the TSA people searching the suitcases the ones making the rules??? 

Its usally the guys (gals) on the front line who catch all the shyte...  :<(  klc

[Steve - WB3HUZ]

Yea, really. I don't think God was invoked for any TSA rules. Your rant is misdirected.

[kb2vxa]

I don't think they care how you pack when you're living at the Piss Weak Hotel.

[Steve - WB3HUZ]

LOL!

[WA3VJB]

huh-HA !
haven't heard that one in a LOONG time

[WD8BIL]

Only here can we go from wierd scope traces..... thru the valley of discontent..... and end up at the Piss Weak Hotel 

AMers...... gotta love'em !

[W1ATR]

lmao, that's friggin awsome.